This invention generally relates to multi-layer ceramic capacitors, and particularly relates to multi-layer ceramic capacitors having high specific resistivities and low dielectric losses at high frequencies, and including inexpensive electrodes.
Recently, multi-layer ceramic capacitors are widely used to meet need of small-sized and large-capacitance capacitors. Conventional electrolytic capacitors generally have large capacitances, but can not operate normally at high frequencies. The multi-layer ceramic capacitors also have large capacitances and can operate normally at high frequencies. Therefore, in high-frequency circuits, multi-layer ceramic capacitors are used at points where large capacitances are necessary. In general, multi-layer ceramic capacitors are produced by a step of co-firing internal electrodes and dielectric ceramics. Ceramic capacitors of the high dielectric constant type conventionally include barium titanate (BaTiO.sub.3) based material. Since the firing temperature of this material is high, e.g., about 1300.degree. C., internal electrodes of the capacitors should be made of metals such as platinum (Pt) and palladium (Pd) which are expensive.
Proposed multi-layer ceramic capacitors use BaTiO.sub.3 based material able to be fired in atmosphere or ambient gas of low partial pressure of oxygen and have internal electrodes made of base metal such as nickel (Ni).
U.S. Pat. No. 3,920,781 discloses a method of manufacturing a multi-layer ceramic capacitor which uses acceptor dopant BaTiO.sub.3 based dielectric material and which has internal electrodes made of a base metal such as nickel (Ni), cobalt (Co), or iron (Fe).
U.S. Pat. No. 4,115,493 discloses a multi-layer ceramic capacitor which uses dielectric material made by doping or adding calcium (Ca) to BaTiO.sub.3 based material. In this case, the ratio A/B is chosen to be 1.005-1.03 where the letter A denotes the sum of the mol ratios of Ba and Ca while the letter B denotes the sum of the mol ratios of the other cations. In addition, this multi-layer ceramic capacitor has internal electrodes made of nickel.
In the known multi-layer ceramic capacitors, since dielectric material consists of BaTiO.sub.3 based material whose firing temperature is higher than the melting point of copper (Cu), base metal such as nickel (Ni) is used as internal electrodes in place of copper. The electrical conductivity of base metal such as nickel (Ni) is lower than that of copper (Cu), silver (Ag), and platinum (Pt). Internal electrodes made of base metal such as nickel (Ni) increase loss factors of related capacitors at high frequencies. Base metal such as nickel has low equilibrium oxygen partial pressure which can be disadvantageous as follows. When dielectric ceramics are fired together with internal electrodes made of base metal such as nickel, the dielectric ceramics are exposed to the equilibrium oxygen partial pressure of the base metal at the firing temperature so that the dielectric ceramics tend to be reduced and to have low resistivities.